1. Field of the Invention
The invention relates to direct current to alternating current power conversion and, more particularly, to photovoltaic module output power conversion to alternating current.
2. Description of the Related Art
An inverter is a device that performs direct current (DC) to alternating current (AC) power conversion. FIG. 1 shows a functional diagram of a prior art inverter 101 implementing the power conversion process, which is described in further detail below.
Inverters can be designed to supply power from photovoltaic (PV) modules to the utility power grid, otherwise simply known as the grid. The process of supplying power to the grid places several special constraints on the power conversion process. First, there exists an optimum voltage across the PV module terminals at which maximum power is to be extracted. This is denoted as the maximum power point and it is found via various measurements and computational algorithms. Second, the utility power grid signal appears as a voltage source with low impedance. The best drive signal from the inverter into the utility power grid is a current. Third, the inverter AC output current must be synchronous with the utility power grid voltage. If it is not synchronous, a non-unity power factor may exist resulting in the transfer of undesirable reactive power or, in an extreme case, no power is efficiently transferred from the inverter into the utility power grid due to a significant frequency or phase difference. Fourth, the inverter must monitor the utility power grid and, if there is a failure of the grid supply, prevent any current flow from the inverter into the grid. Grid failure may be due to a break in the grid wiring to the inverter site. Under this condition, if the inverter drives the grid, the remaining portion of the grid connected to the inverter is energized. Since a limited region of the disconnected grid is now energized, it becomes an island of power relative to the dimensions of the grid. Prevention of the island condition by the inverter grid detection mechanism is known as anti-islanding. Anti-islanding is important in that utility workers can be exposed to the hazard of undesired power in an island and have no means to reliably determine if an island exists or disable the power entering the island, particularly if the grid problem is physically distant from the inverter driving the grid.
Existing photovoltaic inverters generally fall into the category of centralized inverters. The centralized inverter accumulates DC power from multiple PV modules wired in series or series combined with parallel connections to achieve a significant total power. This power is converted to AC within the centralized inverter and is connected to the grid. The expected benefit of this method is that the high DC voltage of a series connected string of PV modules allows for greater efficiency in power conversion. Another benefit is that control and monitoring of the system is also centralized.
There is also a category of distributed inverters in which multiple inverters are used to generate the desired AC power from a number of PV modules. In an extreme case, one inverter can be assigned to convert power from one PV module. If the inverter is mounted on the PV module, the assembly comprising the PV module and inverter is termed an AC module. AC modules are generally connected in parallel as opposed to the series connection typically seen for multiple DC connected PV modules used with a centralized inverter.
The benefits of AC modules are multi-fold. First, if an inverter or PV module fails, all other modules can still provide their full power capacity resulting in minimal impact on the total power produced by a PV system. Second, effects of shading or other means that cause one PV module to operate at reduced current does not affect the operation of other modules. In a centralized system, the series connection means that the PV module with the lowest output current limits the entire string of PV modules to this current, regardless of illumination conditions resulting in an overall loss of power on the order of 10 to 30 percent under typical conditions. Third, the inverter in the AC module is capable of measuring the power output of its associated PV module and, via communications means, can report this data to external devices. Centralized inverter systems require an additional, relatively expensive, sensing and communications system to be mounted at each PV module to be able to monitor the performance of individual PV modules. Other advantages have been documented in the prior art.
Previous attempts at development and marketing of AC modules have met with little or no success. The primary reason has been that the sales volume was too low to achieve any kind of economy of scale. The components used in the associated inverters were off-the-shelf, and in many cases were not optimum for the application. The inverter lifetime was limited by many of these components, especially the electrolytic capacitors used for energy storage. The reliability levels of existing off-the-shelf components used in the inverters has limited their lifetime to between five and ten years.
What is desired, therefore, is a circuit that uses a minimum number of components, uses no limited-reliability components and has been optimized for large-scale manufacturing. Such a device would simultaneously achieve economy of scale to support the rapid adoption of solar power via PV modules and can remain in operation for a period of time consistent with other structural electrical power systems.